Method for achieving a stress profile in a glass

ABSTRACT

A method for generating various stress profiles for chemically strengthened glass. An alkali aluminosilicate glass is brought into contact with an ion exchange media such as, for example, a molten salt bath containing an alkali metal cation that is larger than an alkali metal cation in the glass. The ion exchange is carried out at temperatures greater than about 420° C. and at least about 30° C. below the anneal point of the glass.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/585,247 filed on Sep. 27, 2019, which is a divisional of U.S. patentapplication Ser. No. 14/540,328 filed on Nov. 13, 2014, which claims thebenefit of priority under 35 U.S.C. § 119 of U.S. ProvisionalApplication Ser. No. 61/908,369 filed on Nov. 25, 2013 the content ofeach are relied upon and incorporated herein by reference in theirentirety.

BACKGROUND

The disclosure relates to a method of chemically strengthening a glass.More particularly, the disclosure relates to a method of chemicallystrengthening a glass by ion exchange to generate a stress profilewithin the glass.

Ion exchange has been used to chemically strengthen a glass, providingthe surface of the glass with a compressive layer that is resistant tothe introduction of flaws that cause the glass to crack and break. Theprofile of compressive stress extending from the surface into the glassis typically linearly decreasing or can be approximated by acomplementary error function (erfc).

SUMMARY

The present disclosure provides a method for generating various stressprofiles for chemically strengthened glass. An alkali aluminosilicateglass is brought into contact with an ion exchange media such as, forexample, a molten salt bath containing an alkali metal cation that islarger than an alkali metal cation in the glass. The ion exchange iscarried out at temperatures greater than about 420° C. and at leastabout 30° C. below the anneal point of the glass. A method of forming astress profile in an alkali aluminosilicate glass and an alkalialuminosilicate glass article having an engineered stress profile arealso provided.

Accordingly, one aspect of the disclosure is to provide a method ofstrengthening an alkali aluminosilicate glass. The alkalialuminosilicate glass has an anneal point and comprises a plurality offirst metal cations. The method comprises: immersing the alkalialuminosilicate glass in a molten salt bath comprising at least one saltof a second metal, wherein cations of the second metal are larger thanthe first metal cations; and ion exchanging cations of the second metalfrom the molten salt bath for the first metal cations in the alkalialuminosilicate glass at a temperature of greater than about 420° C. andat least about 30° C. less than the anneal point. The ion exchange formsa region of compressive stress extending from a surface of the glass toa depth of layer of at least about 40 μm into the glass, wherein the acompressive stress at a first depth is at least about 50% of thecompressive stress at the surface of the glass and the first depth beingbetween about 30% and about 70% of the depth of layer.

A second aspect of the disclosure is to provide a method of forming acompressive stress profile in an alkali aluminosilicate glass article.The method comprises immersing the alkali aluminosilicate glass articlein an ion exchange bath at a temperature greater than about 420° C. andat least about 30° C. less than an anneal point of the alkalialuminosilicate glass article, and exchanging a plurality of firstcations in the single ion exchange bath for a plurality of secondcations in the alkali aluminosilicate glass article to form a region ofcompressive stress extending from a surface of the alkalialuminosilicate glass article to a depth of layer of at least about 40μm into the alkali aluminosilicate glass article, wherein a firstcompressive stress at a first depth is at least about 50% of acompressive stress at the surface, the first depth being between about30% and about 70% of the depth of layer.

A third aspect of the disclosure is to provide an alkali aluminosilicateglass article having a region under a compressive stress. The regionextends from a surface of the alkali aluminosilicate glass article to adepth of layer of at least about 40 μm within the alkali aluminosilicateglass article. The alkali aluminosilicate glass article has acompressive stress at the surface and a first compressive stress at afirst depth, the first depth being between about 30% and about 70% ofthe depth of layer, wherein the first compressive stress is at leastabout 50% of the compressive stress at the surface.

These and other aspects, advantages, and salient features will becomeapparent from the following detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an ion exchanged glassarticle;

FIG. 2 is a schematic illustration of compressive stress profiles (i.e.,compressive stress versus depth) obtained by ion exchange;

FIGS. 3 a and 3 b are plots of compressive stress profiles determinedusing the inverse WKB method for samples of a first alkalialuminosilicate glass (glass A) that were ion exchanged under differentconditions;

FIG. 4 is a schematic representation of engineered compressive stressprofiles;

FIG. 5 is a plot of compressive stress profiles determined using theinverse WKB method for samples of a second alkali aluminosilicate glass(glass B) that were ion exchanged under different conditions;

FIG. 6 is a plot of K₂O profiles for glass samples that were ionexchanged under different conditions;

FIG. 7 is a plot of calculated retained strength of ion exchangedsamples of glass;

FIG. 8 is a plot of results of retained strength measurements ofring-on-ring testing after different abrasion pressures on ion exchangedsamples of a first alkali aluminosilicate glass (glass A); and

FIG. 9 is a plot of results of retained strength performance ofring-on-ring testing after different abrasion pressures on ion exchangedsamples of a second alkali aluminosilicate glass (glass B).

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that, unless otherwise specified, termssuch as “top,” “bottom,” “outward,” “inward,” and the like are words ofconvenience and are not to be construed as limiting terms. In addition,whenever a group is described as comprising at least one of a group ofelements and combinations thereof, it is understood that the group maycomprise, consist essentially of, or consist of any number of thoseelements recited, either individually or in combination with each other.Similarly, whenever a group is described as consisting of at least oneof a group of elements or combinations thereof, it is understood thatthe group may contain any number of those elements recited, eitherindividually or in combination with each other. Unless otherwisespecified, a range of values, when recited, includes both the upper andlower limits of the range as well as any ranges therebetween. As usedherein, the indefinite articles “a,” “an,” and the correspondingdefinite article “the” mean “at least one” or “one or more,” unlessotherwise specified. It also is understood that the various featuresdisclosed in the specification and the drawings can be used in any andall combinations.

As used herein, the term “glass article” and “glass articles” are usedin their broadest sense to include any object made wholly or partly ofglass. Unless otherwise specified, all compositions are expressed interms of mole percent (mol %).

It is noted that the terms “substantially” and “about” may be utilizedherein to represent the inherent degree of uncertainty that may beattributed to any quantitative comparison, value, measurement, or otherrepresentation. These terms are also utilized herein to represent thedegree by which a quantitative representation may vary from a statedreference without resulting in a change in the basic function of thesubject matter at issue. Thus, a glass that is “substantially free ofMgO,” for example, is one in which MgO is not actively added or batchedinto the glass, but may be present in very small amounts as acontaminant.

Compressive stress and depth of layer are measured using those meansknown in the art. Such means include, but are not limited to,measurement of surface stress (FSM) using commercially availableinstruments such as the FSM-6000, manufactured by Luceo Co., Ltd.(Tokyo, Japan), or the like, and methods of measuring compressive stressand depth of layer are described in ASTM 1422C-99, entitled “StandardSpecification for Chemically Strengthened Flat Glass,” and ASTM1279.19779 “Standard Test Method for Non-Destructive PhotoelasticMeasurement of Edge and Surface Stresses in Annealed, Heat-Strengthened,and Fully-Tempered Flat Glass,” the contents of which are incorporatedherein by reference in their entirety. Surface stress measurements relyupon the accurate measurement of the stress optical coefficient (SOC),which is related to the birefringence of the glass. SOC in turn ismeasured by those methods that are known in the art, such as fiber andfour point bend methods, both of which are described in ASTM standardC770-98 (2008), entitled “Standard Test Method for Measurement of GlassStress-Optical Coefficient,” the contents of which are incorporatedherein by reference in their entirety, and a bulk cylinder method. Asused herein, “DOL” refers to the depth of the compressive layerdetermined by FSM measurements.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing particular embodiments and are not intended to limit thedisclosure or appended claims thereto. The drawings are not necessarilyto scale, and certain features and certain views of the drawings may beshown exaggerated in scale or in schematic in the interest of clarityand conciseness.

Ion exchange is commonly used to chemically strengthen glasses. In oneparticular example, alkali cations within a source of such cations(e.g., a molten salt, or “ion exchange,” bath) are exchanged withsmaller alkali cations within the glass to achieve a layer that is undera compressive stress (CS) near the surface of the glass. For example,potassium ions from the cation source are often exchanged with sodiumions within the glass. The compressive layer extends from the surface toa depth of layer (DOL) within the glass.

A cross-sectional schematic view of a planar ion exchanged glass articleis shown in FIG. 1 . Glass article 100 has a thickness t, first surface110, and second surface 112. While the embodiment shown in FIG. 1depicts glass article 100 as a flat planar sheet or plate, glass articlemay have other configurations, such as three dimensional shapes ornon-planar configurations. Glass article 100 has a first compressivelayer 120 extending from first surface 110 to a depth of layer d₁ intothe bulk of the glass article 100. In the embodiment shown in FIG. 1 ,glass article 100 also has a second compressive layer 122 extending fromsecond surface 112 to a second depth of layer d₂. Glass article also hasa central region 130 that extends from d₁ to d₂. Central region 130 isunder a tensile stress or central tension (CT), which balances orcounteracts the compressive stresses of layers 120 and 122. The depthd₁, d₂ of first and second compressive layers 120, 122 protects theglass article 100 from the propagation of flaws introduced by sharpimpact to first and second surfaces 110, 112 of glass article 100, whilethe compressive stress minimizes the likelihood of a flaw penetratingthrough the depth d₁, d₂ of first and second compressive layers 120,122.

Typical compressive stress profiles (i.e., compressive stress versusdepth) obtained by ion exchange are schematically shown in FIG. 2 . Inone aspect, the stress profile has a linearly decreasing complementaryerror function shape (represented by line “a” in FIG. 2 ) with acompressive stress at the surface of CS_(a). In another aspect, thecompressive stress profile is linear (represented by line “b” in FIG. 2) with a compressive stress at the surface of CS_(b).

“Engineered” stress profiles that are neither linear nor errorfunction-like have been obtained by two-step, or dual, ion exchangeprocesses, which are typically carried out at temperatures at about 420°C. or less. However, managing the dual ion exchange process iscomplicated, as ion exchange temperature and the Na to K ratio in theindividual ion exchange baths must be carefully managed to avoid surfacetension, and maintenance of two separate ion exchange baths is costprohibitive.

Described herein is a method of generating an engineered stress profilein alkali aluminosilicate glasses and glass articles through an ionexchange of glass over a wide temperature range. Different types ofnon-linear or non-error function profiles may be achieved with the ionexchange process by selecting an ion exchange temperature. Using amechanistic model, it has been found that such engineered profiles canhave higher retained strength and less strength variation than thoselinear or error-function-like profiles that are obtained using dual ionexchange. The observed mechanical advantages are supported byindentation fracture threshold testing and abraded ring on ring testing.Knowing the field failure flaw characteristic in different applications,an optimal stress profile may be designed to achieve optimum productreliability.

Accordingly, methods of ion exchanging and strengthening alkalialuminosilicate glasses are provided. The alkali glass comprises aplurality of first metal cations and has an anneal point. As usedherein, the term “anneal point,” refers to the temperature at which aglass has a viscosity of 10^(13.2) poise.

In a first step, the alkali aluminosilicate glass is brought intocontact with an ion exchange medium that contains a plurality of secondmetal cations. The second metal cation has the same valence/oxidationstate as the first metal cation, and is larger than the first metalcation.

In some embodiments, both the first metal cation and the second metalcation are alkali metal cations. For example, the first metal cation maybe Na⁺ and the second metal cations may be K⁺, Rb⁺, Cs⁺, or anycombinations thereof. In some embodiments, the second metal cation mayinclude other monovalent cations such as Ag⁺, Tl⁺, or the like.

In some embodiments, the ion exchange medium may include a molten orpartially molten salt bath that comprises at least one ionic salt of thesecond metal cation. In those instances in which the second cation isK⁺, for example, the molten salt bath may comprise potassium nitrate(KNO₃), potassium sulfate (K₂SO₄), potassium chloride (KCl), mixturesthereof, or the like. Such salts of the second metal cation typicallyconstitute most of the molten salt bath by weight. The molten salt bathmay also include smaller amounts of salts of the first metal cation aswell as compounds that act to reduce attack of the bath vessel or theglass article by the molten salt. Such additional components mayinclude, but are not limited to, selected components of the glass, suchas silicic acid, alumina in gel form, silica in gel form, or the like.In some embodiments in which the first metal cation is Na⁺ and thesecond cation is K⁺, the molten salt bath comprises at least one ofpotassium sulfate and potassium nitrate and up to about 10% of at leastone sodium salt by weight. In some embodiments, the molten salt bath mayinclude up to about 4% of at least one sodium salt by weight.

In other embodiments, the ion exchange medium may include gels,solutions, sprays, gases, or the like that comprise ionic salts or othercompounds containing the second metal cation.

In those embodiments in which the ion exchange medium is a molten saltbath, the alkali aluminosilicate glass is brought into contact with themolten salt bath by immersion in the bath. The molten salt bath isheated at a temperature that is greater than about 420° C. and is atleast 30° C. less than the anneal point of the alkali aluminosilicateglass. In order to prevent breakage of the glass due to thermal shockand substantial cooling of the molten salt bath, the alkalialuminosilicate glass may be heated prior to immersion in the moltensalt bath. In some embodiments, the glass may be heated prior toimmersion to a temperature that is within about 50° C. of and less thanthe temperature of the molten salt bath.

While the glass article is immersed in the molten salt bath, secondmetal cations from the bath are exchanged for first metal cations in theglass, thus forming a region under a compressive stress, the regionextending from the surface of the glass to a depth of layer. The glassremains immersed in the molten salt bath until a predeterminedcompressive stress level and/or depth of layer is achieved, after whichthe glass is removed from the ion exchange bath and typically washed toremove any residual salts. Actual immersion time also depends on thetemperature at which the ion exchange process is carried out, glasscomposition, and other factors. In some embodiments, ion exchanged timeranges from about 0.5 hours up to about 8 hours. In some embodiments,the depth of layer is at least about 40 microns (μm); in otherembodiments, at least about 50 μm; and in still other embodiments, atleast about 70 μm. In some embodiments, the surface compressive stressCS_(s) is at least about 100 MPa.

Whereas the compressive stress profile achieved in dual ion exchangeprocesses tends to exhibit a maximum or “spike” (e.g., CS_(a), CS_(b) inFIG. 2 ) at the glass surface and decreases either linearly (line “b” inFIG. 2 ) or according to a complementary error function (line “a” inFIG. 2 ), the compressive stress profile achieved using the methoddescribed herein does not generally exhibit a “spike” at the glasssurface. In the present method, the surface stress of ion exchangedglass is significantly lower than that of glass treated at “normal” ionexchange conditions; i.e., single or dual exchange at temperatures of410° C. or less. Compared to glasses ion exchanged at “normal”temperatures of 410° C. or less, glasses that are ion exchanged athigher temperatures exhibit higher compressive stress at a deeper depth,as shown in FIGS. 3 a and 3 b . In one embodiment, at a first depth d₁below the surface, where first depth d₁ is between about 30% and about70% of the depth of layer DOL, the compressive stress is at least about70% of the surface compressive stress. In some embodiments, the firstdepth d₁ is between about 40% and about 60% of the depth of layer, and,in other embodiments, between about 30% and about 35% of the depth oflayer. In certain embodiments, first depth d₁ is about 25 μm. In someembodiments, the compressive stress at first depth d₁ is at least about50% of the surface compressive stress CS_(s); in other embodiments, atleast about 70% of the surface compressive stress CS_(s); and, in otherembodiments, at least about 90% of the surface compressive stressCS_(s).

The ion exchange process described hereinabove, in some embodiments, isthe initial or first ion exchange to which the glass article issubjected. Following the ion exchange process described above, the glassarticle may undergo additional ion exchange. In those instances where ahigh surface compressive stress CS_(s) “spike” is desired, the glassarticle may be ion exchanged in a second bath containing the secondmetal cation at a temperature that is less than that of the first ionexchange bath. In some embodiments, the glass article is ion exchangedin the second bath at a temperature of less than about 420° C. Thesecond bath, in certain embodiments, comprises at least one salt of thesecond metal cation and is substantially free of salts of the firstmetal cation. In the exchange of K⁺ for Na⁺ in the glass article, forexample, the second bath contains only potassium salts and issubstantially free of any sodium salts. While ion exchange time dependson a number of factors, the ion exchange time in the second ion exchangebath is typically less than that in the first ion exchange bath.

In other embodiments, incorporation of a third, larger (i.e., largerthan both the first and second metal cations), metal cation at or nearthe surface of the glass article may be desired to increase thecompressive stress CS_(s) at the surface. Cs⁺ or Rb⁺ ions, for example,may be exchanged for K⁺ ions introduced during the first ion exchange.This may be achieved by ion exchanging the glass article at atemperature that is greater than or equal to the temperature of thefirst ion exchange (i.e., greater than about 420° C. and at least about30° C. less than an anneal point of the alkali aluminosilicate glassarticle) in a second bath comprising the third cation. The second bath,in certain embodiments, comprises at least one salt of the third metalcation and is substantially free of salts of the first and second metalcation. In the exchange of Cs⁺ for K⁺ in the glass article, for example,the second ion exchange bath contains only cesium salts and issubstantially free of any potassium salts. While ion exchange timedepends on a number of factors, the ion exchange time in the second ionexchange bath is typically less than that in the first ion exchangebath.

By increasing the ion exchange temperature, the stress profile may bemanipulated to cover almost the whole CS/depth space available for theglass article. With the specific stress profiles based upon the fracturemechanics framework described hereinbelow, retained strength as afunction of flaw sizes can be predicted.

Most materials tend to fracture when stressed beyond some criticallevel. The stress intensity factor K_(a) is used to predict the stressstate or intensity near the crack tip caused by remote loading or byresidual stresses, which can generally be expressed asK _(a) =Mσ _(a)√{square root over (πs)}  (1)where M is a constant depending on the crack and specimen geometry (hereM=1.12), s is the crack size, and σ_(a) is the applied tensile stress.

The stress intensity factor, K_(r), due to the ion-exchange residualstress profile can be evaluated as

$\begin{matrix}{{K_{r} = {\frac{M}{\sqrt{\pi s}}{\int\limits_{0}^{s}{{\sigma(z)}{g(z)}dz}}}},} & (2)\end{matrix}$where g(z) is the Green function for the crack geometry considered andσ(z) is the engineered stress profile:

$\begin{matrix}{{{g(z)} = \frac{2s}{\sqrt{s^{2} - z^{2}}}}.} & (3)\end{matrix}$

Fracture toughness, K_(IC), is a critical material parameter tocharacterize the material's inherent ability to resist crack growth. Ingeneral, the fracture toughness of glass is taken to be 0.7MPa*m^(−1/2).

The crack starts to grow when the external crack driving force, K_(a),is equal to the internal crack resistance provided by both residualstress profile, K_(r), and the fracture toughness, K_(IC), expressed asK _(a) =K _(IC) −K _(r).  (4)

When K_(r)<0, as in equation (2) discussed above, K_(a) is largeraccording to equation (4). This is the benefit of the residualcompressive stress.

In another aspect, a method of forming a compressive stress profile inan alkali aluminosilicate glass article is provided. The methodcomprises immersing the alkali aluminosilicate glass article in an ionexchange bath, such as those described hereinabove, at a temperaturegreater than about 420° C. and at least about 30° C. less than theanneal point of the alkali aluminosilicate glass article; and exchangingfirst metal cations in the ion exchange bath for second cations in thealkali aluminosilicate glass article to form a region of compressivestress extending from a surface of the alkali aluminosilicate glassarticle to a depth of layer of at least about 40 μm into the alkalialuminosilicate glass article. In one embodiment, the compressive stressat a first depth d₁ below the surface is at least about 70% of thesurface compressive stress. The first depth d₁ is between about 30% andabout 70% of the depth of layer DOL. In some embodiments, the firstdepth d₁ is between about 40% and about 60% of the depth of layer and,in other embodiments, between about 30% and about 35% of the depth oflayer. In certain embodiments, first depth d₁ is about 25 μm. In someembodiments, the compressive stress at first depth d₁ is at least about50% of the compressive stress at the surface, also referred to assurface compressive stress CS_(s); in other embodiments, at least about70% of the surface compressive stress CS_(s); and in other embodiments,at least about 90% of the surface compressive stress CS_(s).

In yet another aspect, an alkali aluminosilicate glass article having anengineered stress profile is provided. The alkali aluminosilicate glassarticle has a region under a compressive stress extending from a surfaceof the article to a depth of layer DOL of at least about 40 μm withinthe glass article. The compressive stress and depth of layer areobtained by those methods described hereinabove. Two non-limitingexamples of such engineered compressive stress profiles areschematically shown in FIG. 4 . Referring to FIG. 4 , the alkalialuminosilicate glass article has a compressive stress CS_(s) at thesurface (also referred to as the surface compressive stress), and afirst compressive stress CS at a first depth d₁. The first depth d₁ isbetween about 30% and about 70% of the depth of layer DOL. In someembodiments, the first depth d₁ is between about 40% and about 60% ofthe depth of layer DOL and, in other embodiments, between about 30% andabout 35% of the depth of layer DOL. In certain embodiments, the firstdepth d₁ is about 25 μm. In some embodiments, the compressive stress CSat the first depth d₁ is at least about 50% of the surface compressivestress CS_(s); in other embodiments, at least about 70% of the surfacecompressive stress CS_(s); and in still other embodiments, at leastabout 90% of the surface compressive stress CS_(s).

In some embodiments, the compressive stress at the surface CS_(s) of thealkali aluminosilicate glass is greater than or equal to the compressivestress CS at the first depth d₁ (example a in FIG. 4 ). In otherembodiments, the compressive stress at the surface CS_(s) of the alkalialuminosilicate glass is less than the compressive stress at the firstdepth d₁ (example b in FIG. 4 ). In some embodiments, the alkalialuminosilicate glass article described herein has a thickness of up toabout 1.5 mm. In other embodiments, the alkali aluminosilicate glassarticle described herein has a thickness in a range from about 0.2 mm upto about 1.5 mm and, in still other embodiments, in a range from about0.2 mm up to about 1.0 mm.

In some embodiments, the alkali aluminosilicate glass comprises at leastabout 50 mol % SiO₂ and at least about 11 mol % Na₂O, and thecompressive stress is at least about 900 MPa. In some embodiments, theglass further comprises Al₂O₃ and at least one of B₂O₃, K₂O, MgO andZnO, wherein−340+27.1·Al₂O₃−28.7·B₂O₃+15.6·Na₂O−61.4·K₂O+8.1·(MgO+ZnO)≥0 mol %. Inparticular embodiments, the glass comprises: from about 7 mol % to about26 mol % Al₂O₃; from 0 mol % to about 9 mol % B₂O₃; from about 11 mol %to about 25 mol % Na₂O; from 0 mol % to about 2.5 mol % K₂O; from 0 mol% to about 8.5 mol % MgO; and from 0 mol % to about 1.5 mol % CaO. Theglass is described in U.S. patent application Ser. No. 13/533,298, byMatthew J. Dejneka et al., entitled “Ion Exchangeable Glass with HighCompressive Stress,” filed Jun. 26, 2012, and claiming priority to U.S.Provisional Patent Application No. 61/503,734, filed Jul. 1, 2011, thecontents of which are incorporated herein by reference in theirentirety.

In other embodiments, the alkali aluminosilicate glass comprises atleast about 50 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂Ocomprises Na₂O; Al₂O₃; and B₂O₃, wherein B₂O₃— (R₂O−Al₂O₃)≥3 mol %. Incertain embodiments, the glass comprises: at least about 50 mol % SiO₂;from about 9 mol % to about 22 mol % Al₂O₃; from about 3 mol % to about10 mol % B₂O₃; from about 9 mol % to about 20 mol % Na₂O; from 0 mol %to about 5 mol % K₂O; and at least about 0.1 mol % MgO, ZnO, orcombinations thereof, wherein 0<MgO<6 and 0<ZnO<6 mol %; and,optionally, at least one of CaO, BaO, and SrO, wherein 0 mol%<CaO+SrO+BaO≤2 mol %. When ion exchanged, the glass, in someembodiments, has a Vickers crack initiation threshold, which isdetermined by application of an indenter load to the surface, of atleast about 10 kgf. Such glasses are described in U.S. patentapplication Ser. No. 13/903,433, by Matthew J. Dejneka et al., entitled“Zircon Compatible, Ion Exchangeable Glass with High Damage Resistance,”filed May 28, 2013, and claiming priority to U.S. Provisional PatentApplication No. 61/653,489, filed May 31, 2012, the contents of whichare incorporated herein by reference in their entirety.

In still other embodiments, the glass comprises at least about 50 mol %SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃,wherein −0.5 mol % Al₂O₃(mol %)−R₂O(mol %)≤2 mol %; and B₂O₃, andwherein B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %)) 4.5 mol %. In otherembodiments, the glass has a zircon breakdown temperature that is equalto the temperature at which the glass has a viscosity of greater thanabout 40 kPoise and comprises: at least about 50 mol % SiO₂; at leastabout 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃; and B₂O₃, whereinB₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %)) 4.5 mol %. In still otherembodiments, the glass is ion exchanged and has a Vickers crackinitiation threshold, determined by application of an indenter load tothe surface, of at least about 30 kgf. In some embodiments, the glasscomprises at least about 50 mol % SiO₂; at least about 10 mol % R₂O,wherein R₂O comprises Na₂O; Al₂O₃, wherein −0.5 mol %≤Al₂O₃(mol%)−R₂O(mol %)≤2 mol %; and B₂O₃, wherein B₂O₃(mol %)−(R₂O(mol%)−Al₂O₃(mol %))≥4.5 mol %. Such glasses are described in U.S.Provisional patent application Ser. No. 13/903,398, by Matthew J.Dejneka et al., entitled “Ion Exchangeable Glass with High DamageResistance,” filed May 28, 2012, and claiming priority to U.S.Provisional Patent Application No. 61/653,485, filed May 31, 2012, thecontents of which are incorporated herein by reference in theirentirety.

In still other embodiments, the alkali aluminosilicate glass comprisesat least about 4 mol % P₂O₅, wherein (M₂O₃(mol %)/R_(x)O(mol %))<1,wherein M₂O₃=Al₂O₃+B₂O₃, and wherein R_(x)O is the sum of monovalent anddivalent cation oxides present in the alkali aluminosilicate glass. Insome embodiments, the monovalent and divalent cation oxides are selectedfrom the group consisting of Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, MgO, CaO, SrO,BaO, and ZnO. In some embodiments, the glass comprises 0 mol % B₂O₃. Theglass is described in U.S. patent application Ser. No. 13/678,013 byTimothy M. Gross, entitled “Ion Exchangeable Glass with High CrackInitiation Threshold,” filed Nov. 15, 2012, and claiming priority toU.S. Provisional Patent Application No. 61/560,434, filed Nov. 16, 2011,the contents of which are incorporated herein by reference in theirentirety.

In some embodiments, the alkali aluminosilicate glasses describedhereinabove are substantially free of (i.e., contain 0 mol % of) of atleast one of lithium, boron, barium, strontium, bismuth, antimony, andarsenic.

In some embodiments, the alkali aluminosilicate glasses describedhereinabove are down-drawable by processes known in the art, such asslot-drawing, fusion drawing, re-drawing, and the like, and has aliquidus viscosity of at least 130 kilopoise.

The following examples illustrate the features and advantages of themethod described herein, and are in no way intended to limit thedisclosure or appended claims thereto.

The chemical strengthening method described herein is illustrated by theion exchange of glass samples having two distinct compositions. Samplescomprising glass A have a nominal composition of 68.96 mol % SiO₂, 0 mol% B₂O₃, 10.28 mol % Al₂O₃, 15.21 mol % Na₂O, 0.012 mol % K₂O, 5.37 mol %MgO, 0.007 mol % Fe₂O₃, 0.006 mol % ZrO₂, and 0.17 mol % SnO₂. Glass Ais described in U.S. patent application Ser. No. 13/533,298, filed Jun.26, 2012, referenced hereinabove. Samples comprising glass B have anominal composition of 67.55 mol % SiO₂, 3.67 mol % B₂O₃, 12.67 mol %Al₂O₃, 13.66 mol % Na₂O, 0.014 mol % K₂O, 2.33 mol % MgO, 0.008 mol %Fe₂O₃, 0.005 mol % ZrO₂, and 0.10 mol % SnO₂. Glass B is described inU.S. patent application Ser. No. 13/903,433, filed May 28, 2013, andclaiming priority to U.S. Provisional Patent Application No. 61/653,489,referenced hereinabove. Neither glass contains Li₂O and/or CaO.

The conditions under which ion exchange were conducted are listed inTables 1 and 2 for glasses A and B, respectively. Salt bath composition,temperature, and ion exchange time were varied for both types ofglasses.

TABLE 1 Conditions used to ion exchange glass A. Bath 1 2 3 4 Ionexchange bath 100% 100% 98.5% 97% composition KNO₃ KNO₃ KNO₃ + KNO₃ +1.5% KSO₄ 3% KSO₄ Ion exchange 410 440 470 500 temperature (° C.) Ionexchange time 6, 9, 13 4, 6, 9 2, 3, 5 1, 2, 3 (hours) Bath 5 6 7 8 Ionexchange bath 95.5% 94% 92.5% 91% composition KNO₃ + KNO₃ + KNO₃ +KNO₃ + 4.5% KSO₄ 6% KSO₄ 7.5% KSO₄ 9% KSO₄ Ion exchange 530 560 590 620temperature (° C.) Ion exchange time 0.5, 1, 2 0.33, 0.67, 0.25, 0.5, 10.17, 0.33 (hours) 1.33

TABLE 2 Conditions used to ion exchange glass B. Bath 1 2 3 Ion exchangebath 100% 95% KNO₃ + 92% KNO₃ + composition KNO₃ 5% KSO₄ 8% KSO₄ Ionexchange 420 530 580 temperature (° C.) Ion exchange time 5.5 1.75 1,1.5, 2 (hours)

Compressive stress profiles of the ion exchanged samples were determinedusing a method for measuring the stress profile based on the TM and TEguided mode spectra of the optical waveguide formed in the ion-exchangedglass. The method includes digitally defining positions of intensityextrema from the TM and TE guided mode spectra, and calculatingrespective TM and TE effective refractive indices from these positions.TM and TE refractive index profiles n_(TM)(z) and n_(TE)(z) arecalculated using an inverse WKB calculation. The method also includescalculating the stress profile S(z)=[n_(TM)(z)−n_(TE)(z)]/SOC, where SOCis a stress optic coefficient for the glass substrate. This method isdescribed in U.S. patent application Ser. No. 13/463,322 by Douglas C.Allan et al., entitled “Systems and Methods for Measuring the StressProfile of Ion-Exchanged Glass,” filed May 3, 2012, and claimingpriority to U.S. Provisional Patent Application No. 61/489,800, filedMay 25, 2011, the contents of which are incorporated herein by referencein their entirety.

The compressive stress profiles were determined using the inverse WKBmethod for samples of glass A that were ion exchanged under differentconditions method, and the results are plotted in FIGS. 3 a and 3 b .When ion exchanged at 410° C. for 9 hours (bath 1 in Table 1), a linearstress profile (FIG. 3 b ) was obtained. Ion exchange at 560° C. for 80minutes (bath 6 in table 1) produces a stress profile (FIG. 3 a ) inwhich the compressive stress at the surface of the glass is lower andlarger amounts of compressive stress are retained at deeper depths belowthe surface.

Compressive stress profiles were determined using the inverse WKB methodfor samples of glass B that were ion exchanged under differentconditions method, and the results are plotted in FIG. 5 . When ionexchanged at 420° C. for 5.5 hours (bath 1 in Table 2), an errorfunction-like stress profile (line “a” in FIG. 5 ) was obtained. Ionexchange at 530° C. for 1.75 hours (bath 2 in Table 2) produced a stressprofile (line “b” in FIG. 5 ) in which a maximum compressive stress ofabout 470 MPa was achieved at a depth of about 10 μm below the surface.After ion exchange for 1.5 hours at 580° C. (bath 3 in Table 2), thecompressive stress profile is essentially constant at about 280 MPa fromthe surface of the glass to a depth of about 35 μm, after which thestress profile gradually decreases to 0 MPa at about 80 μm (line “c” inFIG. 5 ). Ion exchange for 1.5 hours at 560° C. (bath 3 in Table 2)produced a compressive stress profile in which a maximum CS of about 300MPa occurs at a depth of about 30 μm and a depth of layer of about 85 μm(line “d” in FIG. 5 ).

FIG. 6 is a plot of K₂O profiles for samples of glass 1 that were ionexchanged under different conditions. The K₂O profiles were determinedby electron microprobe analysis. The K⁺ ions penetrate into glass toapproximately the same depth as the compressive stress profiles shown inFIGS. 3 a and 3 b.

The retained strength performance versus pre-existing flaw (crack) sizesfrom modeling is shown in FIG. 6 , which is a plot of calculatedretained strength of ion exchanged samples of glass A. The results ofretained strength measurements of samples of glass B that were ionexchanged at 420° C. for 5.5 hours, 530° C. for 1.75 hours, and 580° C.for 1 hour were determined after 2 psi, 5 psi, 7 psi, 10 psi, and 15 psiabrasion are shown in FIG. 7 . The abrasion particles were 90 grit SiC,total volume was 1 ml, and abrasion duration was 5 seconds. After 2 psiabrasion, which corresponds to a shallower flaw depth, glass that wasion exchanged at 420° C. had much higher retained strength than theglasses ion exchanged at higher temperatures. When the abrasion pressureis increased to 5 psi, however, the characteristic retained strength ofglass ion exchanged at 420° C. decreased from 540 MPa to 425 MPa. Theretained strength after 2 psi abrasion for the glass ion exchanged at530° C. is 434 MPa. The value reduced to 410 MPa, where the reduction isonly 24 MPa. As the flaw introduction depth is increased by increasingabrasion pressure from 7 psi to 15 psi, glasses that were ion exchangeat 530° C. and 580° C. show higher retained strength capabilitiescompared to those of standard 420° C. ion exchanged glass. Such retainedstrength performance may also provide advantage in device (i.e., displaywindows and screens for electronic devices) level performance, such asdrop testing.

The advantages provided by the engineered stress profiles describedherein are demonstrated by retained strength performance of ring-on-ringtesting on ion exchanged samples of glass A and glass B. Abrasionparticles were 90 grit SiC, total volume was 1 ml, and abrasion durationwas 5 seconds. The retained strength performance of ring-on-ring testingafter abrasion at different pressures (5 psi, 10 psi, and 15 psi) onglass A samples that were ion exchanged under different conditions isshown in FIG. 8 . The samples were ion exchanged at either 420° C. for5.5 hours, 530° C. for 2.5 hours, or 580° C. for 1.33 hours. Theengineered stress profiles obtained after ion exchange at 530° C. and580° C. provide greater retained strength than the stress profileobtained with ion exchange at 420° C. The retained strength performanceof ring-on-ring testing after abrasion at different pressures (2 psi, 5psi, 7 psi, 10 psi, and 15 psi) on glass B samples that were ionexchanged under different conditions is shown in FIG. 9 . The sampleswere ion exchanged at either 420° C. for 5.5 hours, 530° C. for 1.75hours, or 580° C. for 1 hour. The engineered stress profile obtained byion exchange at 530° C. provides greater retained strength than thestress profile obtained by ion exchange at 420° C. This retainedstrength performance may also provide device level performance, whensubjected to different types of testing such as, for example, as droptesting.

While typical embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the disclosure or appended claims.Accordingly, various modifications, adaptations, and alternatives mayoccur to one skilled in the art without departing from the spirit andscope of the present disclosure or appended claims.

The invention claimed is:
 1. A method of strengthening an alkalialuminosilicate glass comprising first metal cations and having ananneal point, the method comprising: a. immersing the alkalialuminosilicate glass in a molten salt bath, the molten salt bathcomprising at least one salt of a second metal, wherein cations of thesecond metal are larger than the first metal cations; b. ion exchangingthe cations of the second metal from the molten salt bath for the firstmetal cations in the alkali aluminosilicate glass at a temperature ofgreater than about 420° C. and at least about 30° C. less than theanneal point, wherein ion exchanging forms a region of compressivestress, the region extending from a surface of the glass to a depth oflayer of at least 40 μm into the glass, and wherein a compressive stressat the surface is at least about 100 MPa and less than a compressivestress at a first depth, at least a portion of a compressive stressbetween the surface and the first depth is less than the compressivestress at the surface, the first depth being between 30% and 70% of thedepth of layer, and the compressive stress at the first depth is aburied peak.
 2. The method of claim 1, wherein the first metal cationsare sodium cations and the second cations are at least one of potassiumcations, rubidium cations, or silver cations.
 3. The method of claim 2,wherein the molten salt bath comprises at least one of potassium sulfateand potassium nitrate.
 4. The method of claim 3, wherein the molten saltbath further comprises up to about 10 wt % of at least one sodium salt.5. The method of claim 1, wherein the first depth is about 25 μm.
 6. Themethod of claim 5, wherein the compressive stress at a first depth is atleast about 350 MPa.
 7. The method of claim 5, wherein the alkalialuminosilicate glass has a retained strength of at least about 200 MPa.8. The method of claim 5, wherein ion exchanging the alkalialuminosilicate glass comprises ion exchanging the alkalialuminosilicate glass for a period ranging from about 0.5 hours to about8 hours.
 9. The method of claim 1, wherein the glass has a thickness ofup to about 1.5 mm.
 10. The method of claim 1, further comprising: a.immersing the alkali aluminosilicate glass in a second molten salt bath,the second molten salt bath comprising at least one salt of a thirdmetal, wherein cations of the third metal are larger than the firstmetal cations and the cations of the second metal; and b. ion exchangingthe cations of the third metal from the second molten salt bath for atleast one of the first metal cations and the cations of the second metalin the alkali aluminosilicate glass at a temperature of greater thanabout 420° C. and at least about 30° C. less than the anneal point,wherein ion exchanging in the second molten salt bath forms a surfaceregion comprising the cations of the third metal.
 11. The method ofclaim 1, further comprising: a. immersing the alkali aluminosilicateglass in a second molten salt bath, the second molten salt bathcomprising at least one salt of the second metal; and b. ion exchangingthe cations of the second metal from the second molten salt bath for thefirst metal cations in the alkali aluminosilicate glass at a temperatureof less than or equal to about 420° C., wherein the surface compressivestress is formed by ion exchanging in the second molten salt bathcreates a maximum compressive stress at the surface.
 12. The method ofclaim 1, wherein the compressive stress between the surface and thefirst depth is a buried inverted peak.
 13. An alkali aluminosilicateglass article, the alkali aluminosilicate glass article having a regionunder a compressive stress, the region extending from a surface of thealkali aluminosilicate glass article to a depth of layer of at leastabout 40 μm within the alkali aluminosilicate glass article, wherein thealkali aluminosilicate glass article has a compressive stress at thesurface, a compressive stress at a first depth, and a compressive stressbetween the surface and the first depth, the first depth being between30% and 70% of the depth of layer, the compressive stress at the surfaceis at least about 100 MPa and less than the compressive stress at thefirst depth, and the compressive stress at the first depth is a buriedpeak, and at least a portion of the compressive stress between thesurface and the first depth is less than the compressive stress at thesurface.
 14. The alkali aluminosilicate glass article of claim 13,wherein the first depth is about 25 μm.
 15. The alkali aluminosilicateglass article of claim 14, wherein the compressive stress at a firstdepth is at least about 350 MPa.
 16. The alkali aluminosilicate glassarticle of claim 14, wherein the alkali aluminosilicate glass articlehas a retained strength of at least about 200 MPa.
 17. The alkalialuminosilicate glass article of claim 13, wherein the alkalialuminosilicate glass article comprises at least about 50 mol % SiO₂ andat least about 11 mol % Na₂O, and wherein the compressive stress at thesurface is at least about 900 MPa.
 18. The alkali aluminosilicate glassarticle of claim 17, wherein the alkali aluminosilicate glass articlefurther comprises Al₂O₃ and at least one of B₂O₃, K₂O, MgO and ZnO, andwherein −340+27.1·Al₂O₃−28.7·B₂O₃+15.6·Na₂O−61.4·K₂O+8.1·(MgO+ZnO)≥0 mol%.
 19. The alkali aluminosilicate glass article of claim 13, wherein thecompressive stress between the surface and the first depth is a buriedinverted peak.